Immobilizing Pertechnetate in Ettringite via Sulfate Substitution

• DOI: 10.1021/acs.est.0c03119
• Publication Date: 2020-09-10T20:40:01Z
• Authors: Sarah A. Saslow; Sebastien N. Kerisit; Tamas Varga; Sebastian T. Merg...; Claire L. Corkhill; Michelle M. V. Sn...; Nancy M. Avalos; Antonia S. Yorkshire; Daniel J. Bailey; Jarrod Crum; R. Matthew Asmussen
• Keywords: Ettringite; Substitution (logic); Pertechnetate

Technetium-99 immobilization in low-temperature nuclear waste forms often relies on additives that reduce environmentally mobile pertechnetate (TcO4–) to insoluble Tc(IV) species. However, this is a short-lived solution unless reducing conditions are maintained over the hazardous life cycle of radioactive wastes (some ∼10,000 years). Considering recent experimental observations, work explores how rapid formation ettringite [Ca6Al2(SO4)3(OH)12·26(H2O)], common mineral formed cementitious forms, may be used directly immobilize TcO4–. Results from ab initio molecular dynamics (AIMD) simulations and solid-phase characterization techniques, including synchrotron X-ray absorption, fluorescence, diffraction methods, support successful incorporation TcO4– into crystal structure via sulfate substitution when synthesized by aqueous precipitation methods. One one water replaced with OH– during substitution, where Ca2+-coordinated near site deprotonated form for charge compensation upon substitution. Furthermore, AIMD calculations favorable at SO42– rather than gypsum (CaSO4·2H2O, as secondary phase) least 0.76 eV 298 K. These results first their kind suggest contribute overall lifetime performance forms.